GB2128846A - Photographic recording from a video monitor - Google Patents

Abstract

The recording apparatus comprises a video signal input (5a), a video monitor (2,5 to 12) connected thereto for converting the received video signals into images displayed on a screen of the monitor, support means for a photographic medium (4) to be exposed to the images displayed by the monitor (2,5 to 12), and an optical system (3), such as a wide angle lens, operatively interposed between the screen of the video monitor (2,5 to 12) and the photographic medium (4). A compensation circuit (13) is connected to the monitor (2,5 to 12) to modify the brightness of the image displayed in such a way that it complements the illumination characteristics of the optical system (3) whereby the image recorded on the photographic medium (4) exhibits negligible or zero vignetting. The intensity of light transmitted through a wide angle lens varies as COS<4> gamma where gamma is the angle of illumination with respect to the lens axis. Double integration circuits (14,15) generate parabolic signals at line and field rate which are combined and used to control the bias of the control grid (7c). <IMAGE>

Description

SPECIFICATION Apparatus for recording on a photographic medium, images displayed by a video monitor The present invention relates to apparatus for recording, on a photographic medium, images displayed by a video monitor, and in particular to such recording apparatus comprising a video input for receiving video signals, a video monitor connected to said video input and arranged to convert the received video signals into images displayed by said video monitor, and an optical system operatively interposed between the screen of the video monitor and said photographic medium.

Such apparatus, which are also called "video imagers" are used in connection, for example, with electronic medical testing or diagnostic apparatus (ultra sound scanners, computerized tomography apparatus, etc.) which present the result of a diagnostic test as an image on a video monitor. The video signal produced by the apparatus is fed to the input of recording apparatus and converted into images by the monitor inside the recording apparatus so that these images can be recorded on photographic films or plates.

In known image recording apparatus, optical systems are employed which quite often include a wide-angle lens. This kind of lens suffers from the drawback of light intensity fall-off at the edges of the image (vignetting). In fact it is known that these lenses exhibit a fall-off of light from the centre of the lens to its outer edges, which can be as much as 20%. The illumination or transmission characteristics of a lens can in fact be expressed by the following relationship: E0 = Eo cos4y where Ey is the illumination in a direction forming an angle,with respectto the axis of the lens, and Eo represents the on-axis illumination, i.e. the illumination in the centre of the lens.

The cosine4 function can be conveniently approximated buy a parabolic function.

In order to overcome the above drawback it is known to use special compensating optical filters having illumination characteristics substantially complementary to those of the lens. In this manner, it is possible to achieve a drastic reduction of the vignetting effect. However, this solution itself suffers from a drawback in that it causes a general brightness reduction of the resuttant images, and, further, high precision compensating filters are necessary which are very expensive to manufacture.

The object of the present invention is to provide apparatus of the above-specified type, which enables the vignetting of the recorded images to be substantially eliminated in a very simple and economical way that does not require special and expensive precision filters.

This object is achieved, in accordance with the present invention, by the provision of recording apparatus of the aforesaid type which further comprises a compensation circuit connected to the video monitor and arranged to modify the brightness of the image displayed on the screen of said video monitor in a manner complementary to the illumination characteristics of the optical system, whereby the image recorded on the photographic film is subjectto a negligible vignetting effect.

Further characteristics and advantages of the recording apparatus of the invention will become apparent from the description which follows with reference to the appended drawings, provided purely by way of non-limiting example, in which: Figure lisa circuit diagram, partially in block diagram form, of a first embodiment of recording apparatus according to the invention; and Figure 2 is a circuit diagram, also partially in block diagram form, of a second embodiment of recording apparatus to the invention.

With reference to Figure 1, the first embodiment of the recording apparatus comprises a housing 1 in which a video monitor, generally indicated 2, and a lens 3 are disposed. Reference 4 indicates a photographic film or plate to be exposed, through the lens 3, to the image presented on the screen of the monitor 2, whereby to record that image.

The video monitor 2 comprises a conventional input stage 5, comprising preamplification circuits, impedance-matching circuits, etc. The input Saof the input stage 5 represents the input of the apparatus, and is intended to be fed with a video signal from apparatus such as (for example) electronic medical diagnostic apparatus provided with a video monitor for displaying images which represent the result of a test.

The output of input stage 5 is connected to the cathode of a cathode-ray tube 7 through a video amplifier 6.

The input stage 5 is further connected to a sync signal separator stage 8. This stage providesa horizontal sync signal to a first oscillator 9 (horizontal oscillator), and a vertical sync signal to a second oscillator 10 (vertical oscillator).

Oscillator 9 feeds a pulse signal at line frequency to a horizontal-deflection driver stage 11 which is connected to the horizontal deflection coil 7aof cathode-ray tube 7.

The vertical oscillator 10 feeds a pulse signal at frame frequency to a vertical-deflection driver stage 12 which drives the vertical deflection coil 7bof cathode-ray tube 7.

Reference 13 generally indicates a compensation circuit. In the embodiment shown in Figure 1, the compensation circuit comprises two double integrator circuits 14, 15 respectively connected to the output of the horizontal oscillator 9 and to the output of the vertical oscillator 10. In operation, the integrator circuits 14, 15 perform a double integration of the signals coming from the oscillators to which they are respectively connected.

The pulse signal coming from each of the said oscillators is first converted, as a result of the first integration process, into a saw-tooth signal of the same frequency, and then, as a result of the second integration process, into a signal having a parabolic waveform and again of the same frequency.

The outputs of the integrator circuits 14, 15 are connected to the input of a summing circuit 16, which is in turn connected, by means of potentiometer 18 and a capacitor 19, to.the bias circuit 17 of the control grid 7cof the cathode-ray tube 7.

In operation, the signals output from the integrator circuit 14, 15 are added and then superposed upon the bias voltage of the control grid of the cathode-ray tube 7. As a consequence, the brightness of certain of the image elements displayed on the screen of the cathode-ray tube 7 is intensified; this intensification is parabolic, in form, i.e. increases according to a parabolic law from the centre of the image to the outer edges thereof.

By adjusting the integration constants of integrator circuits 14 and 15 (for example, by means of respective potentiometers 20,21) and by adjusting (by means of the potentiometer 18) the amplitude of the signal which is superposed on the bias voltage of the control grid of the cathode-ray tube, it is possible to modify the brightness of the image displayed on the screen of said cathode-ray tube in a way complementary to the illumination characteristics of the lens 3 or other optical system used in the apparatus.

In this way, the image recorded on the photographic film or plate 4 exhibits a substantially negligible vignetting, and a special compensating filter is no longer necessary. Furthermore, the image recorded on plate 4 is not subject to any loss of brightness, unlike the prior art apparatus in which compensating filters were used.

A simplified embodiment of apparatus according to the invention is shown in Figure 2, the components and parts already described with reference to Figure 1 being designated by the same reference numerals as used in that Figure.

In the Figure 2 embodiment, advantage is taken of the fact that the horizontal deflection driver stage 11 usually comprises a voltage step-up transformer 30 with a capacitor 31 connected in parallel with its secondary winding. The current flowing in the primary winding of transformer 30 (deflection current) is proportional to the integral of the pulse signal coming from the horizontal oscillator 9. The capacitor 31 at the output of the transformer 30 effectively performs a second integration of the pulse signal. As a result, the signal present at the secondary winding of the transformer 30 is proportional to the integral of the integral of the signal coming from the horizontal oscillator and can thus be directly fed to the summing circuit 16.

In this case a further integrator circuit 14, such as that shown in Figure 1, is no longer necessary.

The remainder of the circuitry of the Figure 2 embodiment, as well as its operation, are similar to that of the Figure 1 embodiment.

Claims (7)

1. Apparatus for recording, on a photographic medium, images displayed on a video monitor, said apparatus comprising a video input for receiving video signals, a video monitor connected to the said input and arranged to convert the received video signals into images displayed on a screen of the monitor, support means for supporting a photographic medium for exposure to the images displayed by said video monitor, and an optical system operatively interposed between the video monitor and the said photographic medium; said apparatus further comprising a compensation circuit connected to the video monitor and arranged to modify the brightness of the images displayed on the screen of said monitor in a manner complementing the illumination characteristics ofthe optical system whereby the image recorded on the medium is subject two a substantially negligible vignetting effect.

2. Apparatus according to Claim 1, further including a lens having an illumination characteristic curve of the cosine 4y type, where z is the field angle, the said compensating circuit being arranged to increase the relative brightness of each picture element displayed on the video monitor by an amount which is an increasing function of the distance of each picture element from the centre of the video monitor screen.

3. Apparatus according to Claim 2, wherein the said compensation circuit is arranged to increase the relative brightness of each picture element displayed on the screen of the video monitor by an amount which is substantially a parabolic function of the distance of the picture element from the centre of the video monitor screen.

4. Apparatus according to any of the preceding claims, wherein the said video monitor comprises a cathode-ray tube and horizontal line scan and vertical frame scan circuitry including horizontal and vertical oscillators, the said compensation circuit comprising: - a first signal generator circuit arranged to generate a first periodic signal having a parabolic waveform and a frequency equal to the horizontal sync frequency of the video monitor, - a second signal generator circuit arranged to generate a second periodic signal having a parabolic waveform and a frequency which is equal to the vertical sync frequency of the video monitor, and - a summing circuit connected on its input side to the outputs of said first and second signal generating circuits and on its output side to a bias circuit of a control grid of said cathode-ray tube whereby to superpose the sum of said two periodic signals on the bias voltage of said control grid.

5. Apparatus according to Claim 4, wherein the said first and second signal generator circuits comprise respectively first and second double integrator circuits, the first and second integrator circuits being respectively connected to the output of the horizontal oscillator and to the output of the vertical oscillator whereby to integrate twice the pulse signals from said oscillators.

6. Apparatus according to Claim 4, wherein the said first signal generator circuit is constituted by the horizontal deflection circuitry of said cathode-ray tube, the latter circuitry comprising a voltage step-up transformer having a capacitor connected across its secondary winding, whereby in operation a voltage is set up across the said secondary winding which is substantially proportional to the integral of the integral of the pulse signal emitted by the horizontal oscillator of the monitor, the said second signal generator circuit comprising a double integrator circuit which is connected to the output of the vertical oscillator of the monitor and is arranged to integrate twice the pulse signal output from the said vertical oscillator.

7. Recording apparatus substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.